Functional Analysis of Corn Husk Photosynthesis  

Abstract

The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C 3 photosynthetic pathway, in contrast to its leaves, which use the C 4 photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO 2 compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO 2 assimilation rate in the husk was less than that in the leaves and did not saturate at high CO 2 , indicating CO 2 diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO 2 compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C 4 -like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C 4 photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.

Stable Carbon Isotope Discrimination Is under Genetic Control in the C4 Species Maize with Several Genomic Regions Influencing Trait Expression

“…C of the IL was largely decreased compared with what was observed in C 3 plants (Vogel, 1980) and similar to previous reports for maize (Kubásek et al, 2007;Monneveux et al, 2007;Pengelly et al, 2011). Surprisingly, the absolute range for D 13 C within the IL was not much lower than for a diverse set of International Maize and Wheat Improvement Center hybrid cultivars (Tieszen and Grant, 1990) and close to the absolute range reported for C 3 crops (Henderson et al, 1998;Rebetzke et al, 2008;Xu et al, 2008).…”

Ontogeny of the Maize Shoot Apical Meristem

“…Furthermore, leaves initiated from both the SAM and juvenile lateral meristems differ in many ways from husk leaves formed from adult-staged lateral meristems. Compared with foliar leaves, husk leaves have a lower vein density (Langdale et al, 1988), accumulate ribulose-1,5-bis-phosphate carboxylase/ oxygenase in mesophyll as well as bundle sheath cells (Pengelly et al, 2011), and make a greater contribution to grain filling (Fujita et al, 1994;Sawada et al, 1995).…”

C4 Cycles: Past, Present, and Future Research on C4 Photosynthesis

“…In maize, it has been concluded that the C 4 -inducing signals are only perceived in cells that are within a two-cell radius of a vein . This deduction is based on the observation that in leaf-like organs, such as the husk leaf sheath, where up to 20 cells separate vein pairs, dimorphic chloroplast development is only observed in cells immediately surrounding the vasculature (Langdale et al, 1988;Pengelly et al, 2011).…”